The term "stent for angioplasty" is intended generally to indicate devices intended for endoluminal application (for example, in a blood vessel, in association with percutaneous transluminal coronary angioplasty, or PTCA), usually effected by means of catheterisation of a stenotic site dilated using PTCA or ablation. The in situ expansion of the stent stabilises the expanded lumen with a consequent reduction in the likelihood of restenosis, the likelihood of which is very high (approximately 40%) in the case of PTCA alone.
The local support of the lumen by the stent, which is left in its expanded state at the treated site, avoids the restenosis of the site. The use of substantially similar structures to achieve the in situ expansion and anchorage of vascular grafts has already been proposed in the art: naturally, this possible extension of the field of application is also to be understood as coming within the ambit of the invention.
For a general review of vascular stents, reference may usefully be made to the work "Textbook of Interventional Cardiology" edited by Eric J. Topol, W.B. Saunders Company, 1994 and, in particular, to section IV of volume II, entitled "Coronary Stenting".
Many patent documents have also addressed this problem, for example, U.S. Pat. No. 4,776,337, U.S. Pat. No. 4,800,882, U.S. Pat. No. 4,907,336, U.S. Pat. No. 4,886,062, U.S. Pat. No. 4,830,003, U.S. Pat. No. 4,856,516, U.S. Pat. No. 4,768,507, and U.S. Pat. No. 4,503,569.
However, the implantation of these devices may give rise to secondary pathological phenomena such as, for example, acute thrombosis which requires additional antithrombogenic treatment that is usually administered systemically, or an excessive thickening of the neointima (hyperplasia), or spasms of the vascular wall.
Various solutions have been proposed in the past in order to overcome these negative effects.
One line of research generally envisages overcoming the aforesaid negative phenomena by way of radioactive treatments. This first line of investigation is documented, for example, in the following works:
"Intracoronary Radiation Before Stent Implantation Inhibits Neointima Formation in Stented Porcine Coronary Arteries" by Ron Waksman et al., Circulation, 1995; 92; 1383-1386; PA1 "Radioactive Stents for the Prevention of Neointimal Hyperplasia" by Tim A. Fischell, from "The new manual of interventional cardiology", Physician's Press, Birmingham, chapter 18 (1996), p. 134 ss; PA1 "Pure .beta.-Particle-Emitting Stents Inhibit Neointima Formation in Rabbits", by Cristoph Hehrlein et al., Circulation 1996; 93; 641-645; PA1 "Inhibition of Neointimal Proliferation With Low-Dose Irradiation From a .beta.-Paticle-Emitting Stent" by John R. Laird et al., Circulation; 1996; 93; 529-536; and PA1 "The Beta-Particle-Emitting Radioisotope Stent (Isostent): Animal Studies and Planned Clinical Trials" by Tim A. Fischell et al., Am. J. Cardiol. 1996; 78 (suppl. 3A); 45-50. PA1 "Local Drug Delivery: The Development of a Drug Delivery Stenf" by Richard Stack, The Journal of Invasive Cardiology, Vol. 8, No. 8, October 1996 pages 396-397; PA1 "Local Intraluminal Infusion of Biodegradable Polymeric Nanoparticles" by Louis A. Guzman et al., Circulation, 1996; 94; 1441-1448; PA1 "Local Angiopeptin Delivery Using Coated Stents Reduces Neointimal Proliferation in Overstretched Porcine Coronary Arteries" by Ivan De Schreerder et al., J. Invas. Cardiol. 1996; 8; 215-222.
Irrespective of its ultimate effectiveness, this solution encounters an essentially practical difficulty caused by the fact that, in most cases, the use of such a stent assumes the typical features of radiotherapy and/or nuclear medicine. This means that it is necessary to operate in a specifically equipped and authorised environment: this factor has the effect of negating many of the intrinsic advantages of the stent such as, in the first instance, the introduction of techniques once limited to the area of cardiac surgery into much simpler methods of intervention (catheterisation) which can practically be effected at the out-patient level.
Another line of research concerns substantially the administration and/or the localised release of active substances in the zone of the stent. This latter line of research is documented, for example, in the following works:
Many applicational problems are caused by this mode of operation, mainly linked to the specific solutions adopted which are, in any case, related to the fact that it enables little, and maybe even no, flexibility in terms of the timing of the association with the stent of the active substance and/or the possible variation of this latter. In addition, the problem exists of preventing the agent or agents intended for administration in the zone of the stent being delivered or transported to different areas where they could have negative or damaging effects. Other difficulties may arise, for example, in ensuring the permanence and the gradual release over time of active substances capable of being, as it were, washed away by the blood passing through the stent (or the implantation device in general). There are also cases in which it is desirable to be able to supply the site of the implantation device at successive intervals with active substances which, clearly, cannot be applied simultaneously to the device before implantation due to restrictions in their bonding chemistry.
For completeness, reference may also be made to biodegradable stents as illustrated, for example, in the work "Biodegradable Stents: The Future of Interventional Cardiology?" by M. Labinaz et al., Journal of International Cardiology, Vol. 8, No. 4, 1995, pp. 395-405. The main disadvantage of this solution clearly resides in the fact that, at least in the long term when the stent has completely or substantially degraded, it becomes less able mechanically to support the vessel wall against the risk of collapse.